This invention relates to a color demodulation device for use in color television receivers.
In the NTSC system, as is well known, a carrier chrominance signal is multiplexed in frequency-interleaved relation with a luminance signal of 0 to 4.2 MHz. The carrier chrominance signal is produced by quadrature modulation of a color subcarrier with an I signal for orange and cyan having a frequency band of 0 to 1.5 MHz and a Q signal for green and magenta having a frequency band of 0 to 0.5 MHz. The Q signal is transmitted with double sidebands, while the I signal is transmitted with a lower sideband and a vestigial sideband. In the carrier chrominance signal, therefore, Q signal component has a transmission bandwidth of 3.1 MHz to 4.1 MHz, while I signal component has a transmission bandwidth of 2.1 MHz to 4.1 MHz. As stated above, the I signal has more bandwith than the Q signal. The purpose of more bandwith of I signal is to allow more color detail at the receiver for orange and cyan.
In order to eliminate cross-color disturbance, there is provided a signal separation circuit using a comb filter which effectively separates the carrier chrominance and luminance signals. The use of such signal separation circuit allows wideband demodulation of the I signal.
The aforesaid signal separation circuit is composed of a delay circuit (recently made of a charge coupled device or bucket bridge device having a wide passband characteristic) for delaying a composite video signal by one horizontal scanning period (1 H period), an adder circuit connected to receive the input and output signals of the delay circuit for extracting luminance signal component, and a subtracter circuit connected to receive the input and output signals of the delay circuit for extracting carrier chrominance signal component.
Such signal separating function is based on the fact that the luminance signal component and the carrier chrominance signal component are in frequency-interleaved relation, and hence luminance signals on adjacent horizontal lines are in phase, and carrier chrominance signals on the adjacent lines are 180.degree. out of phase. The signal separation is perfectly achieved when a line correlation exists between video signals on two adjacent lines. In the absence of the line correlation, however, the signal separation is imperfect, so that the carrier chrominance signal component appears in the output signal of adder circuit, while luminance signal component appears in the output signal of subtracter circuit. A reproduced picture will suffer dot disturbance due to the carrier chrominance signal component appearing in the output signal of adder circuit, whereas the cross-color disturbance will be caused by the luminance signal component appearing in the output signal of subtracter circuit. Thus, in the absence of the line correlation, the wideband demodulation of I signal for the reproduction of high-quality color picture will increase the cross-color disturbance.